Structure, method and system for complementary strain fill for integrated circuit chips

ABSTRACT

A structure, method and system for complementary strain fill for integrated circuit chips. The structure includes a first region of an integrated circuit having multiplicity of n-channel and p-channel field effect transistors (FETs); a first stressed layer over n-channel field effect transistors (NFETs) of the first region, the first stressed layer of a first stress type; a second stressed layer over p-channel field effect transistors (PFETs) of the first region, the second stressed layer of a second stress type, the second stress type opposite from the first stress type; and a second region of the integrated circuit, the second region not containing FETs, the second region containing first sub-regions of the first stressed layer and second sub-regions of the second stressed layer.

RELATED APPLICATIONS

The present application is a division of U.S. patent application Ser.No. 13/774,069 filed on Feb. 22, 2013 which is a division of U.S. patentapplication Ser. No. 12/983,353 filed on Jan. 3, 2011 now U.S. Pat. No.8,470,674 issued Jun. 25, 2013.

FIELD OF THE INVENTION

The present invention relates to the field of integrated circuits; morespecifically, it relates to structure, method and system forcomplementary strain fill for integrated circuit chips.

BACKGROUND

In modern integrated circuit chips, different regions of the chip mayhave different amounts of strain induced into different regions of thesubstrate in which devices such as field effect transistors arefabricated. As the dimensions of the devices have decreased so has themisalignment tolerance between the mask images of different maskinglevels used to fabricate the various devices and interconnect structuresof the integrated circuit chip. The strain induced into the substratecan often be non-uniform enough across an integrated circuit chip tocause local image placement errors between some of the existingstructures on previously fabricated levels and some of the mask imageson the mask being used to define structures of a current fabricationlevel. Image placement errors can lead to yield loss during fabricationand poor reliability of the completed integrated circuits chips.Accordingly, there exists a need in the art to mitigate or eliminate thedeficiencies and limitations described hereinabove.

SUMMARY

A first aspect of the present invention is a structure, comprising: afirst region of an integrated circuit having a multiplicity of n-channeland p-channel field effect of transistors (FETs); a first stressed layerover n-channel field effect transistors (NFETs) of the first region, thefirst stressed layer of a first stress type; a second stressed layerover p-channel field effect transistors (PFETs) of the first region, thesecond stressed layer of a second stress type, the second stress typeopposite from the first stress type; and a second region of theintegrated circuit, the second region not containing FETs, the secondregion containing first sub-regions of the first stressed layer andsecond sub-regions of the second stressed layer.

A second aspect of the present invention is a method, comprising:forming a first region of an integrated circuit having a multiplicity ofn-channel and p-channel field effect of transistors (FETs); forming afirst stressed layer over n-channel field effect transistors (NFETs) ofthe first region, the first stressed layer of a first stress type;forming a second stressed layer over p-channel field effect transistors(PFETs) of the first region, the second stressed layer of a secondstress type, the second stress type opposite from the first stress type;and forming a second region of the integrated circuit, the second regionnot containing FETs, the second region containing first sub-regions ofthe first stressed layer and second sub-regions of the second stressedlayer.

A third aspect of the present invention is a computer system comprisinga processor, an address/data bus coupled to the processor, andcomputer-readable memory device coupled to communicate with theprocessor, the memory device containing instructions that when executedby the processor implement a method for complementary strain fill forintegrated circuit chips, the method comprising the computer implementedsteps of, comprising: designing an integrated circuit chip; designingtensile and compressive layer etch photomasks; identifying activeregions of an integrated circuit having a multiplicity of n-channel andp-channel field effect of transistors (FETs); identifying inactiveregions of the integrated circuit not containing FETs; placing fillshapes in the tensile and compressive layer etch mask designs, the fillshapes placed only in regions of the tensile and compressive layer etchphotomask designs corresponding to the inactive sub-regions; and storingthe tensile and compressive etch mask designs on a computer readabledevice.

These and other aspects of the invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention are set forth in the appended claims. Theinvention itself, however, will be best understood by reference to thefollowing detailed description of an illustrative embodiment when readin conjunction with the accompanying drawings, wherein:

FIG. 1 is top view of an integrated circuit chip according toembodiments of the present invention;

FIG. 2 is an enlarged view of a sub-region of the integrated circuitchip of FIG. 1 before stressed layers are applied;

FIG. 3 is an enlarged view of a region of a photomask having fill shapescorresponding to the region of the integrated circuit chip illustratedin FIG. 3 according to embodiments of the present invention;

FIG. 4 is an enlarged view of the region of the integrated circuit chipillustrated in FIG. 2 after processing according to embodiments of thepresent invention;

FIG. 5 is a cross-sectional view through line 5-5 of FIG. 4;

FIG. 6 is a plan view of a sub-region of the integrated circuit chipillustrated in FIG. 5;

FIG. 7 is cross-section through line 7-7 of FIG. 6 illustrating atypical field effect transistor;

FIGS. 8A through 8H are cross-sectional views illustrating steps in thefabrication of an integrated circuit chip according to embodiments ofthe present invention;

FIG. 9 is a flowchart of a method for designing photomasks according toembodiments of the present invention;

FIG. 10 is plan view of an actual integrated circuit chip illustratinghow different regions are populated with field effect transistors;

FIG. 11 is an enlarged view of another sub-region of the integratedcircuit chip of FIG. 1 before stressed layers are applied;

FIG. 12 is an enlarged view of a region of a photomask having fillshapes corresponding to the region of the integrated circuit chipillustrated in FIG. 11 according to embodiments of the presentinvention;

FIG. 13 is an enlarged view of the region of the integrated circuit chipillustrated in FIG. 11 after processing according to embodiments of thepresent invention;

FIG. 14 is a cross-sectional view through line 14-14 of FIG. 13;

FIG. 15 is a flowchart of a method for designing photomasks according toembodiments of the present invention; and

FIG. 16 is a schematic block diagram of a general-purpose computer.

DETAILED DESCRIPTION

The present invention embeds layers of opposite stress over inactiveregions of an integrated circuit chip into stressed layers formed overan integrated circuit chip in order to make the strain in differentregions of the integrated circuit chip more uniform. This reduceslocalized errors in photomask (e.g., reticles) alignment as describedinfra. In one example, inactive regions are regions where less than 10%of the area of the region contains active circuit devices (field effecttransistors are examples of active circuit devices). In one example,inactive regions are regions that contain no active circuit devices. Inone example, active regions are regions where greater than 70% of thearea of the region contains active circuit devices.

Stress is a measure of the average amount of force exerted per unitarea. Stress is a measure of the intensity of the total internal forcesacting within a body across imaginary internal surfaces, as a reactionto external applied forces and body forces. Strain is the geometricalexpression of deformation caused by the action of stress on a physicalbody. Young's modulus (E) is a measure of stiffness. It is defined asthe ratio, for small strains, of the rate of change of stress withstrain. In N-channel field effect transistors (NFETs), the mobility ofthe majority carriers, electrons, is greater (hole mobility is less)when the channel is in tensile stress in the direction of current flow.In P-channel field effect transistors (PFETs) the mobility of themajority carriers, holes, is greater (electron mobility is less) whenthe channel region is in compressive stress in the direction of currentflow. Increasing the mobility of majority carriers increases theperformance of the device. Therefore, tensile stressed layers are formedon NFETs and compressive stressed layers are formed on PFETs. Since thedensity of NFETs and PFETs varies locally across an integrated circuitchip, the stress hence the strain also varies locally. Strain can offsetphysical structures from their designed locations. This causes placementerrors as images on subsequent photomasks no longer align to thepreviously defined structures.

A photolithographic process is one in which a photoresist layer isapplied to a surface of a substrate (e.g., integrated circuit chip), thephotoresist layer exposed to actinic radiation through a patternedphotomask (opaque images in a clear field or clear images in an opaquefield) that has been aligned to target marks on the substrate and theexposed photoresist layer developed to form a patterned photoresistlayer. Alignment involves aligning registration images on the photomaskto the target marks on the substrate. After processing (e.g., etching orion implantation), the patterned photoresist is removed. Some images onthe photomask may not register to corresponding images on the integratedcircuit chip because those images have been offset from designedlocations by non-uniform strain as described supra even though theregistration images align perfectly to the target marks.

An example is contact level, which defines the locations of contacts tothe source/drains, gates of FETs. If the source/drains or gates are notwhere they are expected to be based on the design grid because strainhas displaced them, the contact openings in local regions will be etchedin offset locations relative to the source/drains causing some contactsto contact structures they should not contact.

FIG. 1 is top view of an integrated circuit chip according toembodiments of the present invention. In FIG. 1, an integrated circuitchip 100 is divided into regions 105A, 105B, 105C, 105D, 105E, 105F,105G, 105H, 105I, 105J, 105K and 105L. A region where NFETs predominateis defined as a region where greater than a preset target percentage ofall FETs are NFETs. A region where PFETs predominate is defined as aregion where greater than the preset target percentage of all FETs arePFETs. A region where neither NFETs nor PFETs are the predominate typeof FET is defined as a region where the percentage of neither NFETs norPFETs exceed the preset target percentage of all FETs. In one example,the preset target percentage is about 70%. In the example of FIG. 1, inregions 105A, 105B, 105D, 105I, and 105L NFETs predominate (designatedby the label “NFETs”). In regions 105C, 105E, 105H and 105K PFETspredominate (designated by the label “PFETs”). In regions 105F, 105G and105J neither NFETs nor PFETs predominate. A given region can be (i)active or inactive and (ii) NFET predominate or PFET predominate orneither NFET nor PFET predominant. Thus there are six possiblecombinations. To simplify the descriptions of the embodiments of thepresent invention, regions 105A, 105B, 105D, 105I and 105L are activeand predominate NFET regions, regions 105C, 105E, 105H and 105K areactive and predominate PFET regions and regions 105F, 105G and 105J areinactive regions.

Because NFETs predominate in regions 105A, 105B, 105D, 105I, and 105L,the average amount of strain is due to the larger areas of tensilestress layer compared to compressive stress layer in sub-regions 105A,105B, 105D, 105I, and 105L. Because PFETs predominate in sub-region105B, 105E, 105H, and 105K the average amount of strain is due to thelarger areas of compressive stress layer compared top tensile stresslayer in sub-regions 105B, 105E, 105H and 105K. The average strain insub-regions 105F, 105G and 105J will often be lower compared to regions105A, 105B, 105C, 105D, 105E, 105H, 105I, 105K and 105L because neitherareas of compressive nor tensile stress layers predominate to the sameextent as in regions 105A, 105B, 105C, 105D, 105E, 105H, 105I, 105K and105L. Further, the absolute value of stress in tensile stressed layersmay be different than that in compressive stressed layers. The point isstrain is not uniform from region to region or even within sub-regionsof regions 105A, 105B, 105C, 105D, 105E, 105F, 105G, 105H, 105I, 105J,105K and 105L as discussed infra.

The average strain in a region may be roughly determined by

S _(AV) ∝[S _(T) ×A _(T))+(S _(C) +A _(C))]/2  (1)

where

S_(AV) is the average strain in a region;

S_(T) is the amount of stress in the tensile layer;

A_(T) is the area of the tensile stress layer;

S_(C) is the amount of stress in the compressive layer; and

A_(C) is the area of the compressive stress layer.

It will be appreciated in the description that follows, a predominatelyNFET region will be used as an example, but that the invention appliesequally to predominately PFET regions, by substituting PFET for NFET,NFET for PFET, tensile for compressive and compressive for tensile.

FIG. 2 is an enlarged view of region 105A of the integrated circuit chipof FIG. 1 before stressed layers are applied. Region 105A includessub-regions 110A, 110B, 110C and 110D. Sub-regions 110A, 110B and 110Care active regions that include predominately NFETs (there may be PFETspresent as well) and are labeled “ACTIVE”. Sub-region 110D is aninactive region that contains neither NFETs nor PFETs or contains a verysmall numbers of NFETs and/or PFETs, e.g., less than about 10% of thearea of the region contains NFETs and/or PFETs which may be widelyscattered and is labeled “INACTIVE.”

FIG. 3 is an enlarged view of region of a photomask having fill shapescorresponding to the region of the integrated circuit chip illustratedin FIG. 2 according to embodiments of the present invention. In FIG. 3,a photomask region 105AM corresponding to region 105A includessub-regions 110AM, 110BM, 110CM and 110DM corresponding to sub-regions110A, 110B, 110C, 110D of FIG. 2. Region 105M includes fill shapes 115within inactive sub-region 110DM. Fill shapes 115 define areas wheretensile nitride will be removed and compressive nitride formed. There isalso a complimentary photomask similar to the photomask of FIG. 3 but ofopposite polarity having fill shapes that define the extent of thecompressive nitride over sub-region 110D of FIG. 2. See, for example,FIGS. 8A through 8H and description infra.

FIG. 4 is an enlarged view of the region of the integrated circuit chipillustrated in FIG. 2 after processing according to embodiments of thepresent invention. In FIG. 4, a tensile stressed layer 120 and acompressive stressed layer 125 are formed over region 105A. Layer 120extends over regions 110A, 110B and 110C and overlaps region 110D alongthe perimeters of regions 110A, 110B and 110C. Layer 120 may includeregions of compressive stressed layers (e.g., over PFETs) in sub-regions110A, 110B and 110C (see FIG. 6). Layer 125 extends over portions ofsub-region 110D corresponding to fill shapes 115 of FIG. 3. Because ofthe average stress of regions 125 is the opposite of the average stressof regions 120, the average strain in region 105A is lower than wouldotherwise be without region 125 and the strain in region 105A is moreuniform thus reducing image placement errors.

FIG. 5 is a cross-sectional view through line 5-5 of FIG. 4. In FIG. 5,a substrate 130 includes sub-regions 110A and 110B. A dielectric tensilestressed layer 135 has been formed over sub-regions 110A and 110B andoverlapping region 110D. A dielectric compressive stressed layer 140 hasbeen formed over sub-region 110D. Layers 135 and 140 overlap in regions145. There may be regions of layer 140 over sub-regions 110A and 110B(not shown because of the scale of FIG. 5) but as illustrated in FIG. 6.In one example substrate 130 is a single-crystal silicon substrate. Inone example substrate 130 is a silicon-on-insulator (SOI) substratecomprising a single crystal silicon layer separated from a supportingsubstrate by a buried oxide (BOX) layer and regions 105A through 105L ofintegrated circuit chip 100 (see FIG. 1) are formed in the siliconlayer.

FIG. 6 is a plan view of a sub-region of the integrated circuit chipillustrated in FIG. 5. In FIG. 6, sub-region 110A includes NFETs 150Ncovered only by tensile layer 135 and optional PFETs 150P covered onlyby compressive layer 140.

FIG. 7 is cross-section through line 7-7 of FIG. 6 illustrating atypical field effect transistor. In FIG. 7 a typical NFET 150N includesN-type source/drains 155 having N-type source/drain extensions 160 in aP-well 165 of substrate 160 and a gate electrode 170 over a channelregion 175 in P-well 165 between source/drain extensions 175. Gateelectrode 170 is separated from source/drains 155, source/drainextensions 160 and channel region 175 by a gate dielectric 180. Optionaldielectric sidewall spacers 185 are formed on opposite sides of gateelectrode 170. Tensile layer 135 is formed over NFET 150N. NFET 150N isisolated by trench isolation 190 formed in substrate 130. For a PFET,the source/drains and source/drain extensions would be P-type and formedin an N-well.

FIGS. 8A through 8H are cross-sectional views illustrating steps in thefabrication of an integrated circuit chip according to embodiments ofthe present invention. In FIG. 8A, sub-regions 110A, 110B and 110D havebeen formed in region 105A substrate 130. Sub-regions 110A and 110Binclude, for example, PFETs, NFETs and trench isolation. but aredominated by NFETs.

In FIG. 8B, tensile stressed layer 135 is formed on sub-regions 110A,110B and 110D. In one example, tensile stressed layer 135 is siliconnitride (Si₃N₄). In one example, a tensile stressed Si₃N₄ layer isformed by low-pressure chemical vapor deposition (LPCVD) using silane(SiH₄) and ammonia (NH₃) precursor gases. In one example, tensilestressed layer 135 is between about 50 nm and about 100 nm thick. In oneexample, the amount of tensile stress is between about 0.5 GPa and about4 GPa.

In FIG. 8C, a patterned photoresist layer 200 is formed on tensilestressed layer 135 having an opening 205 over sub-region 110D. Opening205 was defined by fill shape 115 of FIG. 3.

In FIG. 8D, tensile stressed layer 135 is removed (e.g., by wet orreactive ion etch (RIE)) to form a trench 210 completely throughstressed layer 135 over sub-region 110D.

In FIG. 8E, patterned photoresist layer 200 (see FIG. 8D) is removed.

In FIG. 8F, compressive stressed layer 140 is formed on tensile stressedlayer 135 and on sub-region 110D in trench 210. In one example,compressive stressed layer 140 is Si₃N₄. In one example, a compressivestressed Si₃N₄ layer is formed by high density plasma (HDP) depositionor plasma enhanced chemical vapor deposition (PECVD) using SiH₄, NH₃ andnitrogen (N₂) precursor gases. In one example, a compressive stressedlayer 140 is between about 60 nm and about 120 nm thick. In one example,the amount of compressive stress is between about 0.5 GPa and about 4GPa.

In FIG. 8G, a patterned photoresist layer 215 is formed on compressivestressed layer 140 having over sub-region 110D. Patterned photoresistlayer 215 was defined by a complimentary (opposite polarity) photomaskhaving a similar photomask fill shape to that of photomask fill shape115 of FIG. 3.

In FIG. 8H, compressive stressed layer 140 is removed (e.g., by wet orRIE) where not protected by patterned photoresist layer 215 (see FIG.8G) to form an island 220 of compressive stressed layer 140 onsub-region 110D. Patterned photoresist layer 215 (see FIG. 8G) is alsoremoved.

It will be apparent to one of ordinary skill in the art that the orderof deposition and patenting of tensile layer 135 and compressive layer140 may be reversed, however deposition and patterning of tensilestressed films is preferred.

FIG. 9 is a flowchart of a method for designing photomasks according toembodiments of the present invention. In step 250, the complimentaryphotomasks of an integrated circuit design that define the extents ofthe tensile and compressive layers are designed.

In step 255, regions of the integrated circuit design where NFETs (andthus tensile stress) predominate and where PFETs (and thus compressivestress) predominate are identified. The photomask data from step 250 maybe used or other design data that defines locations of NFETs and PFETsmay be used.

In step 260, the regions of step 255 are grouped into active sub-regions(sub-regions that contain FETs) and inactive sub-regions (sub-regionsthat do not contain FETs) as illustrated in FIG. 2.

In step 265, fill shapes are inserted into the inactive regions of thecompressive and tensile layer etch mask designs as illustrated in FIG.3. Fill shapes are placed in inactive regions of the tensile layer etchphotomask design when NFETs predominate and in inactive regions of thecompressive layer etch photomask design when PFETs predominate. Fillshapes are placed in inactive regions of the compressive layer etchphotomask design when PFETs predominate and in inactive regions of thetensile layer etch photomask design when NFETs predominate. Fill shapesdefine areas of stress layers to be removed.

In step 270, the tensile and compressive photomask etch mask designs arestored on a computer readable storage medium or device as maskfabrication datasets (e.g., netlists and/or shapes files). Netlist andshapes files impart a logical and physical structure to the storagemedium as specific data for fabricating specific structures are locatedin specific locations on the medium. Actual tensile and compressivelayer etch photomasks may be fabricated using the mask fabricationdatasets.

FIG. 10 is plan view of an actual integrated circuit chip 300illustrating how different regions are populated with field effecttransistors. In FIG. 10, dark areas are NFETs and clear regions arePFETs. An exemplary region 305 is dominated by NFETs. An exemplaryregion 310 is dominated by PFETs. An exemplary region 315 is notdominated by NFETs or PFETs. An exemplary region 320 contains neitherNFETs or PFETs or contains a very small number of NFETs and/or PFETs,e.g., less than about 10% of the area of the region contains NFETs orPFETs) which may be widely scattered.

For regions having no NFETs or PFETs or very small numbers of NFETs andPFETs, a slightly different method is required as is now explained.

FIG. 11 is an enlarged view of region 105G of the integrated circuitchip of FIG. 1 before stressed layers are applied. Region 105G containsneither NFETs or PFETs or contains a very small number of NFETs and/orPFETs, e.g., less than about 10% of the area of the region containsNFETs or PFETs) which may be widely scattered. In the initial integratedcircuit design, region 105G may be entirely compressive stressed orentirely tensile stressed. However, if there are NFETs in region 105G,there may be a tensile stress layer over the NFETs and a compressivestress layer over the rest of region 105G so compressive stress is thepredominate stress layer in the region. Likewise, if there are PFETs inregion 105G, there may be a compressive stress layer over the PFETs anda tensile stress layer over the rest of region 105G so tensile stress isthe predominate stress layer in the region. Compressive and tensilestress layers may comprise silicon nitride.

FIG. 12 is an enlarged view of region of a photomask having fill shapescorresponding to the region of the integrated circuit chip illustratedin FIG. 11 according to embodiments of the present invention. In FIG.12, a photomask region 105GM corresponding to region 105G having fillshapes 350 is shown. When region 105G is initially designed with atensile stress layer (or a tensile stress layer predominates), fillshapes 350 define areas where the tensile layer will be removed and acompressive layer formed. There is also a complimentary photomasksimilar to the photomask of FIG. 12 but of opposite polarity having fillshapes that define the extent of the compressive layer over region 105Gof FIG. 11.

When region 105G is initially designed compressive stress layer (or acompressive stress layer predominates), fill shapes 350 define areaswhere the compressive layer will be removed and a tensile layer formed.There is also a complimentary photomask similar to the photomask of FIG.12 but of opposite polarity having fill shapes that define the extent ofthe tensile layer over sub-region 105G of FIG. 11.

FIG. 13 is an enlarged view of the region of the integrated circuit chipillustrated in FIG. 11 after processing according to embodiments of thepresent invention. In FIG. 13, oppositely stressed sub-regions 355 andsub-region 360 have been formed in region 105G. Because of the averagestress of sub-region 355 is the opposite of the average stress ofsub-region 360, the average strain in region 105G is lower than wouldotherwise be without sub-regions 355 and the strain in region 105G ismore uniform thus reducing image placement errors.

FIG. 14 is a cross-sectional view through line 14-14 of FIG. 13. In FIG.14, a dielectric first type stressed layer 355S has been formed oversub-regions 355 and overlapping sub-region 360. A dielectric second andopposite type stressed layer 360S has been formed over sub-region 360.Layers 355S and 360S overlap.

FIG. 15 is a flowchart of a method for designing photomasks according toembodiments of the present invention. In step 365, the complimentaryphotomasks of an integrated circuit design that define the extent of thetensile and compressive layers are designed.

In step 370, the integrated circuit design is divided into active andinactive regions. The photomask data from step 365 may be used or otherdesign data that defines areas of no FETs or having low density FETs maybe used.

In step 375, the regions of step 370 are grouped into tensilesub-regions and compressive sub-regions.

In step 380, fill shapes are inserted into the inactive regions of thecompressive and tensile layer etch mask designs as illustrated in FIG.12. Fill shapes define areas of stress layers to be removed.

In step 385, the tensile and compressive photomask etch mask designs arestored on a computer readable storage medium or device as maskfabrication datasets (e.g., netlists and/or shapes files). Actualtensile and compressive layer etch photomasks may be fabricated usingthe mask fabrication datasets.

It should be understood that the method described relative to FIGS. 9and 15 may be practiced together on the same integrated circuit chipdesign or just one of the methods may be practiced.

Thus the embodiments of the present invention provide a structure havingmore uniform strain and method for fabricating structures with moreuniform strain so as to reduce or eliminate strain induced imageplacement errors.

Generally, the method described herein with respect to designingphotomasks for complementary strain fill for integrated circuit chips ispracticed with a general-purpose computer and the methods describedsupra in the flow diagrams of FIG. 9 and FIG. 15 may be coded as a setof instructions on removable or hard media for use by thegeneral-purpose computer.

FIG. 16 is a schematic block diagram of a general-purpose computer. InFIG. 16, computer system 400 has at least one microprocessor or centralprocessing unit (CPU) 405. CPU 405 is interconnected via a system bus410 to a random access memory (RAM) 415, a read-only memory (ROM) 420,an input/output (I/O) adapter 425 for connecting a removable data and/orprogram storage device 430 and a mass data and/or program storage device435, a user interface adapter 440 for connecting a keyboard 445 and amouse 450, a port adapter 455 for connecting a data port 460 and adisplay adapter 465 for connecting a display device 470.

ROM 420 contains the basic operating system for computer system 400. Theoperating system may alternatively reside in RAM 415 or elsewhere as isknown in the art. Examples of removable data and/or program storagedevice 430 include magnetic media such as floppy drives and tape drivesand optical media such as CD ROM drives. Examples of mass data and/orprogram storage device 435 include electronic, magnetic, optical,electromagnetic, infrared, and semiconductor devices. Examples of acomputer-readable medium include a semiconductor or solid state memory,magnetic tape, a removable computer diskette, a random access memory(RAM), a read-only memory (ROM), a rigid magnetic disk and an opticaldisk. Current examples of optical disks include compact disk-read onlymemory (CD-ROM), compact disk-read/write (CD-R/W) and DVD. In additionto keyboard 445 and mouse 450, other user input devices such astrackballs, writing tablets, pressure pads, microphones, light pens andposition-sensing screen displays may be connected to user interface 440.Examples of display devices include cathode-ray tubes (CRT) and liquidcrystal displays (LCD).

A computer program with an appropriate application interface may becreated by one of skill in the art and stored on the system or a dataand/or program storage device to simplify the practicing of thisinvention. In operation, information for the computer program created torun the present invention is loaded on the appropriate removable dataand/or program storage device 430, fed through data port 460 or typed inusing keyboard 445.

Thus, the embodiments of the present invention provide a method fordesigning photomasks for complementary strain fill for integratedcircuit chips.

The description of the embodiments of the present invention is givenabove for the understanding of the present invention. It will beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of various modifications,rearrangements and substitutions as will now become apparent to thoseskilled in the art without departing from the scope of the invention.Therefore, it is intended that the following claims cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A computer system comprising a processor, anaddress/data bus coupled to said processor, and computer-readable memorydevice coupled to communicate with said processor, said memory devicecontaining instructions that when executed by the processor implement amethod for complementary strain fill for integrated circuit chips, saidmethod comprising the computer implemented steps of, comprising:generating an integrated circuit chip design for an integrated circuitchip; generating a tensile layer etch mask design and a compressivelayer etch mask design from said integrated circuit chip design;identifying active regions of an integrated circuit having amultiplicity of n-channel and a multiplicity of p-channel field effecttransistors (FETs); identifying inactive regions of said integratedcircuit design not containing FETs; placing fill shapes in said tensileand compressive layer etch mask designs, said fill shapes placed only inregions of said tensile and compressive layer etch mask designscorresponding to said inactive regions; and storing said tensile andcompressive etch mask designs on a computer readable device.
 2. Thecomputer system of claim 1, the method further including: identifyingsub-regions of said active regions, said sub-regions not containingFETs; and placing additional fill shapes in said tensile and compressivelayer etch mask designs, said additional fill shapes placed only inregions of said tensile and compressive layer etch mask designscorresponding to said sub-regions such that when PFETs are thepredominate type of FET in said active regions a tensile layer willresult on said integrated circuit chip fabricated from said tensile andcompressive layer etch mask designs and when NFETs are the predominateFETs of said sub-regions a tensile layer will result on said integratedcircuit chip fabricated from said tensile and compressive layer etchmask designs.
 3. The computer system of claim 1, the method furtherincluding: placing first additional fill shapes in said tensile layeretch mask design, said additional fill shapes placed only in regions ofsaid tensile layer etch mask design where NFETs predominate; and placingsecond additional fill shapes in said compressive layer etch maskdesign, said additional fill shapes placed only in regions of saidcompressive layer etch mask design where PFETs predominate.
 4. Themethod of claim 1, wherein said fill shapes define areas of stresslayers to be removed from said integrated circuit chip fabricated saidtensile layer etch mask design and said compressive layer etch maskdesign.
 5. The method of claim 1, wherein greater than 70% of the areaof each active region of said integrated circuit design contains FETs.6. The method of claim 1, wherein said fill shapes are placed in saidtensile layer etch mask design designs corresponding to said inactiveregions only when greater than 70% of the area of said correspondingactive regions of said integrated circuit design contains NFETs.
 7. Themethod of claim 1, wherein said fill shapes are placed in saidcompressive layer etch mask design designs corresponding to saidinactive regions only when greater than 70% of the area of saidcorresponding active regions of said integrated circuit design containsPFETs.
 8. The method of claim 1, wherein said tensile layer etch maskdesign and said compressive layer etch mask design are stored in anetlist.
 9. The method of claim 1, wherein said tensile layer etch maskdesign and said compressive layer etch mask design are stored in ashapes file.
 10. The method of claim 1, wherein said computer readablestorage device is selected from the group consisting of magnetic tape, aremovable computer diskette, a random access memory device, a read onlymemory device, a rigid magnetic disk and an optical disk.
 11. A computersystem comprising a processor, an address/data bus coupled to saidprocessor, and computer-readable memory device coupled to communicatewith said processor, said memory device containing instructions thatwhen executed by the processor implement a method for complementarystrain fill for integrated circuit chips, said method comprising thecomputer implemented steps of, comprising: generating an integratedcircuit chip design for an integrated circuit chip; generating a tensilelayer etch mask design and a compressive layer etch mask design fromsaid integrated circuit chip design; identifying active regions of anintegrated circuit having a multiplicity of n-channel and a multiplicityof p-channel field effect transistors (FETs); identifying low FETdensity regions of said integrated circuit design, said low FET densityregions containing less than 10% FETs by area; placing fill shapes insaid tensile and compressive layer etch mask designs, said fill shapesplaced only in regions of said tensile and compressive layer etch maskdesigns corresponding to said low FET density regions; and storing saidtensile and compressive etch mask designs on a computer readable device.12. The computer system of claim 11, the method further including:identifying sub-regions of said active regions, said sub-regions notcontaining FETs; and placing additional fill shapes in said tensile andcompressive layer etch mask designs, said additional fill shapes placedonly in regions of said tensile and compressive layer etch mask designscorresponding to said sub-regions such that when PFETs are thepredominate type of FET in said active regions a tensile layer willresult on said integrated circuit chip fabricated from said tensile andcompressive layer etch mask designs and when NFETs are the predominateFETs of said sub-regions a tensile layer will result on said integratedcircuit chip fabricated from said tensile and compressive layer etchmask designs.
 13. The computer system of claim 11, the method furtherincluding: placing first additional fill shapes in said tensile layeretch mask design, said additional fill shapes placed only in regions ofsaid tensile layer etch mask design where NFETs predominate; and placingsecond additional fill shapes in said compressive layer etch maskdesign, said additional fill shapes placed only in regions of saidcompressive layer etch mask design where PFETs predominate.
 14. Themethod of claim 11, wherein said fill shapes define areas of stresslayers to be removed from said integrated circuit chip fabricated saidtensile layer etch mask design and said compressive layer etch maskdesign.
 15. The method of claim 11, wherein greater than 70% of the areaof each active region of said integrated circuit design contains FETs.16. The method of claim 11, wherein said fill shapes are placed in saidtensile layer etch mask design designs corresponding to said low FETdensity regions only when greater than 70% of the area of saidcorresponding active regions of said integrated circuit design containsNFETs.
 17. The method of claim 11, wherein said fill shapes are placedin said compressive layer etch mask design designs corresponding to saidlow FET density regions only when greater than 70% of the area of saidcorresponding active regions of said integrated circuit design containsPFETs.
 18. The method of claim 11, wherein said tensile layer etch maskdesign and said compressive layer etch mask design are stored in anetlist.
 19. The method of claim 11, wherein said tensile layer etchmask design and said compressive layer etch mask design are stored in ashapes file.
 20. The method of claim 11, wherein said computer readablestorage device is selected from the group consisting of magnetic tape, aremovable computer diskette, a random access memory device, a read onlymemory device, a rigid magnetic disk and an optical disk.